The present invention relates to a soldering alloy, a cream solder for soldering electronic circuit boards, and a process of soldering same.
Size reduction and high-density mounting of electronic components are fast-growing trends in electronic parts assembly technology as presently practiced. With this trend, there is an increasing demand for higher reliability and performance of soldering materials especially as they relate to narrow-pitched electronic parts. Meanwhile, there is an increasing concern for environmental protection and there is a movement legally to regulate disposal of industrial wastes, including electronic circuit boards.
A conventional soldering material is depicted in the drawings. FIG. 2 shows an alloy composition of a conventional soldering material and its metallic structure in a bonding interface between a copper land and a solder. In FIG. 2, reference numeral 1 indicates an xcex1-solid-solution Sn layer. 2 indicates xcex2-solid-solution Pb layer. 3 indicates an intermetallic compound composed of Cu3Sn. 4 indicates an intermetallic compound composed of Cu6Sn5. 5 indicates a Cu land.
The conventional soldering material described above is a eutectic alloy composed of Sn and Pb, having a eutectic point of 183xc2x0 C., in which Sn accounts for 63 weight % and Pb 37 weight % of the alloy. The alloy contains the xcex1 solid solution 1 and the xcex2 solid solution 2 in lamellar states. Further, the intermetallic compounds 3 and 4 are formed in the bonding interface between the copper land and the solder.
From the environmental protection view point, however, there is a world-wide rapidly spreading movement to restrict the use of lead (a toxic substance) in Snxe2x80x94Pb alloy solders. The conventional solder material in which the alloy has a lamellar structure presents a problem because the alloy components swell at high temperatures causing in turn the solder to be stressed. As a result, the interfaces between the alloy components are caused to slide, resulting in soldering cracks. Another problem is that, during soldering or at high temperatures, a rigid and brittle two-layered intermetallic compound is produced in the bonding interface between the solder and the copper land, causing cracks to be formed in the interface.
In view of the problems discussed above, it is an object of the present invention to provide a soldering material free from lead, in which alloy components are fine structured and intermetallic compounds are restrained from growing in the bonding interface between a solder and a copper land, so that the changes of the components with the passage of time at high temperatures can be minimized and an excellent thermal fatigue resistance at high temperatures can be obtained.
In order to achieve the aforesaid object, a soldering alloy according to a first aspect of the invention contains Sn, Ag and Bi as the main components, wherein the content of Sn is 83 to 92 weight %, that of Ag is 2.5 to 4.0 weight % and that of Bi is 5 to 18 weight %.
In order to achieve the aforesaid object, a soldering alloy according to a second aspect of the invention contains Sn, Ag, Bi, Cu and In as the main components, wherein the content is 80 to 92 weight %, that of Ag is 2.4 to 4.0 weight %, that of Bi is 5 to 18 weight %, that of Cu is 0.1 to 0.7 weight % and that of In is 0.1 to 1.5 weight %.
According to the first aspect, the soldering alloy is composed mainly of Sn, and addition of a small quantity of Ag thereto enables the tin to provide an alloy having a fine alloy structure and an excellent thermal fatigue resistance at high temperatures. Further, addition of a small quantity of Bi to the alloy enables the compound to lower the melting point and improve the wettability of the alloy.
According to the second aspect, the soldering alloy composed of the same components as of the first aspect further contains a small quantity of Cu, whereby the growth of intermetallic compounds in the bonding interface between the copper land and the solder can be restrained. Further, adding a small quantity of In can improve the elongation property and thermal fatigue resistance of the alloy.
The reason the soldering alloy composition is so limited as mentioned above according to the first and second aspects of the invention is explained below.
Ag is effective to improve the thermal fatigue resistance of the alloy. However, if the quantity of added Ag is less than 2.5 weight %, such an effect cannot be sufficiently displayed. Meanwhile, in order to have the melting point not exceed 220xc2x0 C., the Ag content has to be 4.0 weight % or less. If the added quantity of Ag exceeds 4.0 weight %, the melting point unfavorably rises rapidly. For this reason, the quantity of added Ag should preferably be kept between 2.5 and 4.0 weight %.
Bi is effective to lower the melting point and to enhance the wettability of the alloy. However, if the quantity of added Bi is less than 5 weight % (based upon the weight of the alloy), such effects cannot be sufficiently displayed. Moreover, if the quantity of added Bi exceeds 18 weight % (based upon the weight of the alloy), adequate soldering strength cannot be obtained. Therefore, the quantity of addition of Bi preferably should be between 5 and 18 weight % based upon the weight of the alloy.
Cu is effective to improve the thermal properties of the alloy and to restrain the formation of intermetallic compounds in the bonding interface between the solder and the copper land. However, if the quantity of Cu added is less than 0.1 weight %, such effect does not occur, and if the quantity of Cu exceeds 0.7 weight %, the alloy becomes rigid and brittle. Therefore, the quantity of addition of Cu is preferably between 0.1 and 0.7 weight % based upon the weight of the alloy.
Adding In effectively improves the elongation property, wettability and the thermal fatigue resistance of the alloy. However, if the quantity of added In is less than 0.1 weight %, such effects do not take place, and if the quantity exceeds 1.5 weight %, the mechanical strength of the alloy is decreased. Therefore, the quantity of addition of In should be preferably between 0.1 to 1.5 weight % based upon the weight of the alloy.
A cream solder according to a third aspect of the present application is prepared by adding and mixing a flux into the alloy of the first or second aspects, and has features similar to those of the solder of the first or second aspects of the invention.
In order to achieve the aforesaid object of the invention, a soldering method according to a fourth aspect of the invention involves solidifying the soldering alloy of the first or second aspects of the invention by quenching, so that the intermetallic compounds can be dispersed finely and the mechanical strength of the alloy is increased.
An applicable quenching method preferably involves a cold air blowing, using a quenching rate of between 5 to 15xc2x0 C./second, most preferably at or about 10xc2x0 C./second.
According to the fourth aspect of the invention, quench solidification of the soldering alloy allows the intermetallic compounds of Ag3Sn, Cu3Sn, and CuSn5 to be restrained from growing and to be dispersed finely, whereby the mechanical strength and thermal fatigue resistance of the alloy can be improved.
A fifth aspect of the invention concerns a soldering alloy containing Sn, Ag, Bi and Cu as the main components wherein the Sn content is 77 to 92 weight %, the Ag content is 2.0 to 4.0 weight %, the Bi content is 5 to 18 weight %, and the Cu content is 0.1 to 1.0 weight %. Further related aspects include a cream solder containing the soldering alloy of the fifth aspect of the invention and a soldering method similar to that described above but using the soldering alloy according to the fifth aspect of the invention.
A sixth aspect of the invention concerns a soldering alloy containing Sn, Ag, Bi and In as the main components wherein the Sn content is 76 to 92 weight %, the Ag content is 2.0 to 4.0 weight %, the Bi content is 5 to 18 weight %, and the In content is 0.5 to 4.5 weight %. Further related aspects include a cream solder containing the soldering alloy and a soldering method using same.
A seventh aspect of the invention concerns a soldering alloy containing Sn, Ag, and Bi as the main components wherein the Sn content is 79 to 93 weight %, the Ag content is 2.0 to 3.0 weight %, and the Bi content is 5 to 18 weight % wherein the soldering alloy is solidified by quenching to finely disperse intermetallic compounds causing a resultant increase in the mechanical strength of the soldering alloy.
Yet further aspects of the invention are described herein.